1. Field of the Invention
The present invention relates to slow-maturing, determinate pea plants with improved growth and processing traits.
2. Description of Related Art
The garden pea (Pisum sativum L.) is an important crop plant, with many uses. Seeds of the pea plant, or peas, are consumed in either a dry mature form or in an immature state. The dry mature seed is used for human consumption and as animal feed. Seeds harvested in their immature form are commonly referred to as fresh peas, or garden peas, and are used as fresh vegetables or processed, typically by canning or freezing.
Peas have also proven to be useful experimental plants. Peas are normally self-pollinating, as the carpels and stamens are enclosed within the petals of the plant's flowers, and are therefore naturally pure breeding. In the 1860s, Gregor Mendel discovered the basic rules of inheritance by making various pea crosses and examining the phenotypic ratios of F2 progeny. Among other things, Mendel identified a locus called rugosus that is expressed, as a recessive trait, as a wrinkled appearance of the seed; presence of the dominant allele, R, leads to round seeds. This locus was later re-named r (White, Proc. Am. Phil. Soc. 56, 487-588 (1917)). Bhattacharyya et al. ((1990) Cell 60:115-122) demonstrated that the r locus encodes for a starch branching enzyme.
Simple sugars, such as fructose, glucose and sucrose, are sweet tasting to humans, while more complex carbohydrates, like starch, are not. Biochemical conversion of simple sugars to starch occurs in plants in both the cytosol and in the plastid, via 10-12 enzymatic steps. Any mutant that slows the conversion of sugars to starch can result in a phenotype of sweeter tasting immature peas and wrinkled seed.
Of particular note is the rb locus, which encodes the large subunit of the adenosine disphosphate glucose pyrophosphorylase gene, abbreviated ADPGPP. The ADPGPP enzyme is a heterotetramer in plants, with two small and two large subunit components. The rb mutant, when present as a homozygote (rb/rb) causes a decreased enzyme activity and slows starch accumulation. The rb mutant is also known as the alsweet gene, due to its effect on taste.
Peas with recessive genotypes at the r and rb loci (r/r and rb/rb) produce seeds that are typically sweeter than peas with only one recessive mutation (r/r or rb/rb).
In addition to the r and rb loci, researchers have found a number of other mutants in the starch synthesis pathway. Five wrinkled-seed loci that affect the starch content of the seed are r and rb, plus rug3, rug4 and rug5. The locus named low amylose (lam) influences starch, but not seed shape. The positions of these mutations in the pathway have been described (Wang et al. (1998) J. Exp Bot 49:481-502). rug3, rug4, rug5 and lam correlate to various mutations in genes for phosphoglucomutase, sucrose synthase, starch synthase for amylase and starch synthase for amylopectin, respectively. All of these mutants decrease the starch content of the seed to varying degrees by adversely affecting the activity of enzymes in the biosynthetic pathway through which sugars are converted to starch, thereby leading to sweeter peas. (Hedley, C. L., et al. (2002) Starch/Stärke 54: 235-42).
Determinate and indeterminate growth habits are available in peas, differing in whether or not the plants set a terminal flowering node. As only the first two or three reproductive nodes of indeterminate peas actually contribute to yield of fresh peas, pods set at the fourth node or higher do not contribute to the crop, and are generally discarded as waste.
In contrast, determinate peas set a terminal flowering node, with two racemes of pods, at the second or third reproductive node. Compared to indeterminate varieties, which set pods at nodes that do not contribute to yield, determinate varieties are expected to have a higher yield potential, from the elimination of wasted resources, and could be expected to produce more uniform peas. Determinate varieties should also be easier to harvest than indeterminate varieties because they stand more upright and bear pods at the top of the plant.
One drawback to developing determinate varieties for fresh pea production has been the perception that such peas mature more rapidly than indeterminate types, and that the harvest window of the immature and still sweet peas would be very short, i.e., the varieties are “flashy”. Flashiness, or rapid maturity, tends to create difficulties for growers, especially where weather conditions or simultaneous maturity of multiple fields may prevent harvesting during the relatively narrow available window. This perception has limited the use of the determinate trait by pea breeders; for instance, no one has developed a commercial determinate variety with dark green immature peas.
Another drawback to determinate peas is that the original source of determinacy (det) in peas is tightly linked to the dominant Pa allele, which results in immature seeds that are light green. Consumers strongly prefer dark green peas, making a determinate light green variety virtually unmarketable under present conditions. The Pa and det loci are tightly linked on linkage group V. In order to develop determinate dark green peas that would be suitable for processing and the market, the close linkage between Pa and det must be broken, and a pa det/pa det genotypic combination created through a recombinational event. This event is rare between loci so tightly linked.
There remains a need, then, for fresh processing pea plants having high consumer acceptance for color and sweetness, and good horticultural characteristics such as the ease of harvest provided by determinate peas, but with a maturity rate permitting a harvest window comparable to that of indeterminate commercial pea plants.